This invention relates generally to turbomachines such as gas turbine engines and, more particularly, to an improved fastener shield for minimizing temperature rise associated with protrusions in a fluid flow path.
U.S. Pat. Nos. 4,190,397 and 5,090,865, assigned to the assignee of the present invention, each describe the need for and use of fastener shields, referred to therein as “windage shields”, in gas turbine engines. In particular, the efficiency of the engine is directly related to the ability of the engine to operate at higher turbine inlet temperatures. The need for higher turbine operating temperatures requires cooling air to be supplied to various components of the engine in order to allow the components to operate at the higher temperatures without being subjected to thermal stress to a degree that is damaging to the engine.
In order to supply cooling air at a temperature that is effective to lower the temperature of the operating components, cooling air is extracted from a compressor section of the engine and routed through various channels to the turbine section. As the cooling air is subjected to work input in passing through these channels, the temperature of the cooling air rises. Elements that have been found to significantly affect work in the cooling fluid flow are nuts and bolt heads utilized in connecting various sections of the turbine together. These fastener elements protrude into the cooling air channels creating aerodynamic drag, causing heating of the cooling fluid in a manner that the cooling air receives more work.
The U.S. Patents referenced above describe fastener shields that improve the performance of gas turbine engines. The fastener shields described therein are particularly useful with flange connections that protrude into the fluid flow passage and are connected together by bolts with heads in the fluid flow passage.
The fastener shield described in the '397 patent includes a continuous ring having a generally L-shaped profile that is captured between the bolt head and an upstream flange. The captured flange portion of the shield is provided with a plurality of circumferentially spaced, milled slots contoured to receive D-shaped bolt heads. These bolt heads are mounted flush with the upstream captured portion of the shield, thus eliminating open access holes and protruding bolts. The combination of D-shaped heads and contoured slots provides a means for torquing the bolts.
The cylindrical section of the L-shaped shield extends downstream of the mating flanges and passes the nut side of the bolted connection to direct cooling air past the nut, thereby minimizing velocity reduction from the nut, and represented a distinct improvement over prior art flange connections, such as shown in FIG. 3 of the '397 patent.
While the fastener shield as described in the '397 patent is effective to reduce drag effects within the fluid flow channel of a gas turbine engine, a plurality of contoured slots must be machined in the surface of the fastener shield facing the fluid flow path so that the heads of the bolts fit into the precision machined slots of the shield. Furthermore, the described fastener shield has an L-shaped cross-section with a portion which extends parallel to the direction of fluid flow within the fluid flow channel with the described intent of directing the main fluid flow past bolt heads on the opposite side of the bolted flange.
However, this extended portion does not eliminate flow over the bolt heads due to secondary circulating fluid fields. Thus, it was desirable to have a fastener shield which did not extend into the fluid flow channel and which did not require the specialty-designed bolt heads or a plurality of precision machined slots for receiving the bolt heads, and which accommodates secondary fluid flows.
The '865 patent thus provides a continuous ring of substantially rectangular cross-section formed with a plurality of circumferentially spaced, arcuate-shaped grooves on a first surface of the ring that are oriented so that the ring may be positioned over the bolt heads within the grooves of the ring. A plurality of apertures formed through the ring are aligned with the apertures in the spaces between adjacent grooves. Each of the apertures has a countersunk portion on an outward side of the ring opposite the side containing the grooves.
At least some of the bolts connecting the flanges together extend through the ring at the apertures for holding the ring in position over the bolt heads. The bolts extending through the ring have heads that are recessed into the countersunk areas, with the top of the bolt heads lying flush with the outer surface of the ring.
The countersunk portions fit snugly around the bolt heads to minimize the area of any cavity which could be exposed and lead to disturbance in the fluid flow path. The ring is designed so that when placed in its operative position over the bolt heads, the lower surface of the ring in which the grooves are formed fits snugly against the flange and one edge of the ring also abuts the annular member to which the flange is attached. Fluid is thus prevented from passing under the fastener shield.
The present invention provides further advantages over the above-described fastener shields by further reducing the temperature through the high pressure turbine forward shaft area.
This is accomplished by separating the fastener shield from the compressor discharge pressure (CDP) seal. This permits the fastener shield to be removed without removing the CDP seal, and allows the fastener shield to thermally expand separately from the CDP seal, thus maintaining sealing performance of the CDP seal over a longer period of time.